This invention relates to a color picture tube with stripe shaped phosphors, and more particularly to a color picture tube wherein a terrestrial magnetism compensating or correcting member is contained in the envelope of the tube.
In a conventional color picture tube, due to the effect of terrestrial magnetism the electron beams are deviated from their normal paths so that they do not land on the correct positions on the fluorescent screen with the result that the color purity of the reproduced picture is impaired thus degrading the quality of the picture.
For this reason, in a large size color picture tube, for the purpose of preventing terrestrial magnetism from affecting the electron beams, a magnetic shielding member is disposed on the outer wall or inside of the tube but in most of the small size color picture tubes it has been impractical to provide such magnetic shielding member from the standpoint of cost and construction.
The relationship between terrestrial magnetism and the landing of the electron beam on the fluorescent screen will first be considered.
The horizontal intensity of the terrestrial magnetism is the largest at or near the equator but it decreases gradually toward the north and south magnetic poles of the earth, while the relation becomes contrary as to the vertical intensity of the terrestrial magnetism. For example, the vertical component of the terrestrial magnetism which greatly affects the color picture tube with stripe shaped phosphors varies from 0 to 0.7 gauss between the equator and the poles. The relationship between the vertical component of the terrestrial magnetism and the beam landing is shown in FIG. 1 in which the outer frame represents the size of the picture. In this example, the tube has a size of 12 inches. The data represents the error at portions where the data are written, and the arrows show the direction in which error occurs, the horizontal direction in this example. Taking 0.35 gauss as an example, which represents the average value of the vertical component in Japan, and taking as a reference a case wherein the effect of the horizontal terrestrial magnetism is zero (0 gauss) the landing error of various portions of the picture is one half of that of FIG. 1 as shown by FIG. 2. Where the landing error at the center of the picture is adjusted to be zero by a purity adjusting correction magnet mounted on the outside of the neck portion of the envelope in a manner well known in the art, the landing error can be reduced by 28.5 microns throughout the picture, as shown in FIG. 3.
However, as can be noted from FIG. 3, in a color picture tube not provided with terrestrial magnetism compensating means, although it is possible to reduce to zero the landing error at the center of the picture by the purity adjusting correction magnet, it is impossible to reduce to zero the landing error on both sides of the center. This is due to the fact that the length of travel of the electron beams increases toward the periphery of the picture so that the electrons are more liable to be affected by the terrestrial magnetism thus increasing the deviation of the electron beams. This landing error increases at higher latitude areas.
In addition to the vertical component, the horizontal component of the terrestrial magnetism also causes landing error in the axial direction. The landing error in Japan caused by the horizontal component is the largest when the tube axis is directed in the south to north direction, as shown in FIG. 4. Accordingly, in a conventional color picture tube landing errors are caused by both the vertical and horizontal components of the terrestrial magnetism.
For the purpose of decreasing the beam landing error caused by the terrestrial magnetism throughout the picture, it has been proposed to include a terrestrial magnetism correction member in the tube.
In one example shown in FIG. 5, a quadilateral frustum shaped shielding member 15 is mounted on the back of a supporting frame 11 of a shadow mask assembly 10, the shielding member having a base aligned with the periphery of the funnel 12a of an envelope 12. Reference numeral 13 represents a shadow mask. Such construction is disclosed, for example, in U.S. Pat. No. 3,549,932.
Although this construction can substantially decrease the adverse effect of the terrestrial magnetism in comparison to a conventional color picture tube not provided with such shielding member, there arises the following problems.
Since the size of the magnetic shielding member is the same or larger than the size of the shadow mask a large space is necessary for the surface treatment of the magnetic shielding member, thus decreasing the treating capacity. Furthermore, during the operation of the tube, the temperature of the shadow mask increases due to the scanning of the electron beams, but as the inner surface or the impinged surface of the shadow mask is bounded by the shielding member, heat radiation is prevented thus causing deformation of the shadow mask. Moreover, with this construction it is necessary to increase the size of the demagnetizing coil wound about the envelope near the fluorescent screen for the purpose of demagnetizing the shadow mask, since a substantial portion of the demagnetizing flux flows through the shielding member instead of the shadow mask. This results in a more incomplete demagnetization of the shadow mask than with a tube not using the shielding member. Accordingly, to demagnetize the shadow mask as desired, it is necessary to increase the demagnetizing current. For the reason described above, use of an internal shield for correcting the terrestrial magnetism as shown in FIG. 5 results in such defects as increasing the manufacturing difficulty and degradation of the tube characteristics. These disadvantages are more serious in a small size color picture tube.